A direct radiating loudspeaker typically has a set of transducers, i.e., drivers, on the baffle, i.e., front panel, of the speaker enclosure and directly face an intended audience. Ideally, the soundwaves from these drivers emanate in the direction of the intended audience. Directivity measures the directional characteristic of the soundwaves. Directivity indicates how much sound is directed toward a specific area compared to all of the sound energy being generated by a sound source. Loudspeakers with a high directivity, i.e., propagating in a particular direction and not in other directions, can be heard clearer by the intended audience. In a reverberant space, loudspeakers with low directionality, i.e., propagating in all directions, only contribute to the reverberant field. The conventional loudspeaker takes a “shotgun” approach, scattering sound in an uncalculated manner across the room. High frequency sound reverberates off the floors and ceilings, resulting in an imperfect sound. Note, however, that low frequency sounds, such as bass, are omni-directional. Omni-directional sounds disperse in every direction. Adding more speakers may lower the directionality and make the sound volume and quality even worse.
A line array of equally spaced similar drivers may exhibit a more narrow radiation pattern or beamwidth, in a plane containing the line and normal to the baffle in which the drivers are mounted, than a single driver. The higher frequency sounds emanating from a loudspeaker consists of a main lobe and side lobes. Beamwidth is measured as the included angle of one-quarter power (−6 dB) points of the main lobe projection. A smaller beamwidth angle is directly proportional to higher directivity. Without corrective filtering, the beamwidth of a line array becomes increasingly narrower with increasing frequency. The frequency at which the narrowing of the beamwidth begins to occur is a function of the length of the line array.
There are several problems with the narrowing of the beamwidth. One problem is that the beamwidth, in the plane of the line array, is not constant as a function of frequency. Another problem is that a large number of radiating elements or drivers, must be used in order to obtain a line array with sufficient length to get directivity control of a sufficiently low frequency. Conventional devices using line arrays have not sufficiently addressed these problems.
U.S. Pat. No. 4,363,115 to Cuomo discloses a method for determining optimum element spacing for a low frequency, log-periodic acoustic line array comprising a plurality of omnidirectional hydrophones arranged in a line wherein the spacing between the hydrophones is based on a logarithmic relationship using multiple dipole pairs, each pair centered about the acoustic axis of the array, such that the distance between each dipole pair bears a constant ratio to the wavelength of the acoustic frequency band to be investigated by that hydrophone pair. However, each hydrophone pair operates within a preselected frequency band, exclusive from the other hydrophone pairs.
U.S. Pat. No. 4,653,606 to Flanagan discloses an electroacoustic device with broad frequency range directional response. The array comprises a set of equispaced transducer elements with one element at the center and an odd number of elements in each row and each column. The device uses second order, i.e., 12 dB per octave, filtering of the transducer elements. Beamwidth variations are minimized over the desired frequency range by decreasing the size of the array as the incident sound frequency increases. This is realized by reducing the number of active receiver elements as frequency increases, starting with the extremities of the array. However, the second order filtering of equispaced transducer elements does not provide ideal loudspeaker directivity.
U.S. Pat. No. 6,128,395 to De Vries discloses a loudspeaker system with controlled directional sensitivity. The loudspeakers have a mutual spacing, which, insofar as physically possible, substantially corresponds to a logarithmic distribution, wherein the minimum spacing is determined by the physical dimensions of the loudspeakers used. The frequency dependent variation is inversely proportional to the number of loudspeakers per octave band and is 50% for a distribution of one loudspeaker per octave. However, the logarithmic spacing and delay function does not provide ideal loudspeaker directivity.
A desired loudspeaker arrangement minimizes the number of drivers needed by optimizing the spacing of the drivers and driving function for consistent directivity.